ATP-modulated K+ channels sensitive to antidiabetic sulfonylureas are present in adenohypophysis and are involved in growth hormone release

ATP sensitive potassium channel openers: A new class of ocular hypotensive agents

ATP sensitive potassium (K_ATP) channels connect the metabolic and energetic state of cells due to their sensitivity to ATP and ADP concentrations. K_ATP channels have been identified in multiple tissues and organs of the body including heart, pancreas, vascular smooth muscles and skeletal muscles. These channels are obligatory hetero-octamers and contain four sulfonylurea (SUR) and four potassium inward rectifier (K_ir) subunits. Based on the particular type of SUR and K_ir present, there are several tissue specific subtypes of K_ATP channels, each with their own unique set of functions. Recently, K_ATP channels have been reported in human and mouse ocular tissues. In ex vivo and in vivo model systems, K_ATP channel openers showed significant ocular hypotensive properties with no appearance of toxic side effects. Additionally, when used in conjunction with known intraocular pressure lowering drugs, an additive effect on IOP reduction was observed. These K_ATP channel openers have also been reported to protect the retinal ganglion cells during ischemic stress and glutamate induced toxicity suggesting a neuroprotective property for this drug class. Medications that are currently used for treating ocular hypertensive diseases like glaucoma do not directly protect the affected retinal cells, are sometimes ineffective and may show significant side effects. In light of this, K_ATP channel openers with both ocular hypotensive and neuroprotective properties, have the potential to develop into a new class of glaucoma therapeutics.

Effect of sulfonylureas administered centrally on the blood glucose level in immobilization stress model

Sulfonylureas are widely used as an antidiabetic drug. In the present study, the effects of sulfonylurea administered supraspinally on immobilization stress-induced blood glucose level were studied in ICR mice. Mice were once enforced into immobilization stress for 30 min and returned to the cage. The blood glucose level was measured 30, 60, and 120 min after immobilization stress initiation. We found that intracerebroventricular (i.c.v.) injection with 30 µg of glyburide, glipizide, glimepiride or tolazamide attenuated the increased blood glucose level induced by immobilization stress. Immobilization stress causes an elevation of the blood corticosterone and insulin levels. Sulfonylureas pretreated i.c.v. caused a further elevation of the blood corticosterone level when mice were forced into the stress. In addition, sulfonylureas pretreated i.c.v. alone caused an elevation of the plasma insulin level. Furthermore, immobilization stress-induced insulin level was reduced by i.c.v. pretreated sulfonylureas. Our results suggest that lowering effect of sulfonylureas administered supraspinally against immobilization stress-induced increase of the blood glucose level appears to be primarily mediated via elevation of the plasma insulin level.

Vascular endothelial growth factor secretion from pituitary folliculostellate cells: role of KATP channels

Vascular endothelial growth factor (VEGF) is an endothelial cell mitogen responsible for physiological and pathological angiogenesis. Abnormal regulation of VEGF expression in anterior pituitary folliculostellate (FS) cells has been implicated in pituitary tumour progression. FS and endocrine cells express VEGF, which is considered to be secreted by the constitutive pathway. The present study investigated the mechanism of VEGF secretion in TtT/GF cells, a mouse FS cell line. TtT/GF cells were shown to express VEGF(164), the most potent and bioavailable isoform of VEGF. Immunofluorescence and immunogold electron microscopy localised VEGF to the cytoplasm and small electron-lucent vesicles. Pituitary adenylate cyclase-activating polypeptide (PACAP), a well-documented stimulant of VEGF secretion, caused a robust increase in VEGF secretion over 24 h. Glyburide, an ABCA1 and K(ATP) channel blocker, also caused an increase in VEGF secretion when applied alone, and amplified the response to PACAP. Other ABCA1 transport blockers did not affect VEGF secretion. Exposure of TtT/GF cells to cycloheximide with PACAP or glyburide inhibited the increased secretion of VEGF, consistent with control of secretion at the transcription level. The SUR2B/Kir6.1 form of K(ATP) channels was shown to be expressed by TtT/GF cells. Diazoxide, a K(ATP) activator, inhibited PACAP- and PACAP + glyburide-stimulated VEGF secretion but not that of glyburide alone. These data suggest that K(ATP) channels are expressed by FS cells and play a significant role in the control of VEGF secretion, and also that activation of K(ATP) channels inhibits the secretion of VEGF at the level of transcription.

Neuroprotective role of ATP-sensitive potassium channels in cerebral ischemia

ATP-sensitive potassium (K(ATP)) channels are weak, inward rectifiers that couple metabolic status to cell membrane electrical activity, thus modulating many cellular functions. An increase in the ADP/ATP ratio opens K(ATP) channels, leading to membrane hyperpolarization. K(ATP) channels are ubiquitously expressed in neurons located in different regions of the brain, including the hippocampus and cortex. Brief hypoxia triggers membrane hyperpolarization in these central neurons. In vivo animal studies confirmed that knocking out the Kir6.2 subunit of the K(ATP) channels increases ischemic infarction, and overexpression of the Kir6.2 subunit reduces neuronal injury from ischemic insults. These findings provide the basis for a practical strategy whereby activation of endogenous K(ATP) channels reduces cellular damage resulting from cerebral ischemic stroke. K(ATP) channel modulators may prove to be clinically useful as part of a combination therapy for stroke management in the future.

Sulfonylureas: do we need to introspect safety again?

INTRODUCTION

Sulfonylureas (SUs) are the most commonly prescribed medications for type 2 diabetes mellitus worldwide. Differences among SUs for kinetic and adenosine triphosphate sensitive potassium (KATP) channels selectivity and consequential extrapancreatic effects, although recognized in literature, are not considered by treatment guidelines.

AREAS COVERED

The roles of SUs in various system-related adverse effects have not been well understood. Inconsistencies in the literature and lack of clinical trials assessing the long-term effects of monotherapy or combination therapy with SUs add to the concern. This review provides insights in issues concerning safety of SUs based on literature published between 1980 and 2011. A comprehensive search was carried out on PubMed, Embase and Cochrane databases using the search terms: sulfonylureas, sulfonylureas and KATP channels, sulfonylureas and cardiovascular (CV) effects and sulfonylureas side effects.

EXPERT OPINION

SUs have been linked to CV events, growth hormone (GH) disorder, malignancy, weight gain, erectile dysfunction and central nervous system (CNS) adverse effects. These adverse effects generally get masked as they are thought to be related to diabetes per se. The current article will allow the fraternity to ponder and undertake further research on the ill effects of largely prescribed antidiabetic medication.

Cardiovascular, metabolic and hormonal responses to the progressive exercise performed to exhaustion in patients with type 2 diabetes treated with metformin or glyburide

OBJECTIVES

To evaluate the effects of Metformin and Glyburide on cardiovascular, metabolic and hormonal parameters during progressive exercise performed to exhaustion in the post-prandial state in women with type 2 diabetes (T2DM).

DESIGN AND METHODS

Ten T2DM patients treated with Metformin (M group), 10 with Glyburide (G group) and 10 age-paired healthy subjects exercised on a bicycle ergometer up to exercise peak. Cardiovascular and blood metabolic and hormonal parameters were measured at times -60 min, 0 min, exercise end, and at 10 and 20 minutes of recovery phase. Thirty minutes before the exercise, a standard breakfast was provided to all participants. The diabetic patients took Metformin or Glyburide before or with meal.

RESULTS

Peak oxygen uptake (VO(2)) was lower in patients with diabetes. Plasma glucose levels remained unchanged, but were higher in both diabetic groups. Patients with diabetes also presented lower insulin levels after meals and higher glucagon levels at exercise peak than C group. Serum cortisol levels were higher in G than M group at exercise end and recovery phase. Lactate levels were higher in M than G group at fasting and in C group at exercise peak. Nor epinephrine, GH and FFA responses were similar in all 3 groups.

CONCLUSION

Progressive exercise performed to exhaustion, in the post-prandial state did not worsen glucose control during and after exercise. The administration of the usual dose of Glyburide or Metformin to T2DM patients did not influence the cardiovascular, metabolic and hormonal response to exercise.

Regulation of KATP channel subunit gene expression by hyperglycemia in the mediobasal hypothalamus of female rats

The ATP-sensitive potassium (K(ATP)) channels are gated by intracellular adenine nucleotides coupling cell metabolism to membrane potential. Channels comprised of Kir6.2 and SUR1 subunits function in subpopulations of mediobasal hypothalamic (MBH) neurons as an essential component of a glucose-sensing mechanism in these cells, wherein uptake and metabolism of glucose leads to increase in intracellular ATP/ADP, closure of the channels, and increase in neuronal excitability. However, it is unknown whether glucose and/or insulin may also regulate the gene expression of the channel subunits in the brain. The present study investigated whether regulation of K(ATP) channel subunit gene expression might be a mechanism by which neuronal populations adapt to prolonged changes in glucose and/or insulin levels in the periphery. Ovariectomized, steroid-replaced rats were fitted with indwelling jugular catheters and infused for 48 h with saline, glucose (hyperglycemia-hyperinsulinemia), insulin and glucose (hyperinsulinemia), diazoxide (control), or glucose and diazoxide (hyperglycemia). At the end of infusions, the MBH, preoptic area, and pituitary were dissected for RNA isolation and RT-PCR. Hyperglycemia decreased Kir6.2 mRNA levels in the MBH in both the presence and absence of hyperinsulinemia. These same conditions also produced a trend toward decreased SUR1 mRNA levels in the MBH; however, it did not exceed statistical significance. Hyperglycemia increased whereas hyperinsulinemia reduced neuropeptide Y mRNA levels when these groups were compared with each other. However, neither was significantly different from values observed in saline-infused controls. In conclusion, hyperglycemia per se may alter expression of K(ATP) channels and thereby induce changes in the excitability of some MBH neurons.

The effects of metformin and glibenclamide on glucose metabolism, counter-regulatory hormones and cardiovascular responses in women with Type 2 diabetes during exercise of moderate intensity

AIMS

To compare the effects of metformin and glibenclamide on cardiovascular, metabolic and hormonal parameters during exercise of moderate intensity performed in the postprandial state, in women with Type 2 diabetes.

METHODS

Ten patients treated with metformin, 10 with glibenclamide and 10 control subjects (C) exercised on a bicycle ergometer at 50% of oxygen uptake (VO(2)) peak for 45 min. Cardiovascular, blood metabolic and hormonal parameters were determined at times -60 min (fasting), 0, +15, +30, +45 min (exercise) and at +60, +90 min (recovery). Thirty minutes prior to exercise, participants consumed a standard breakfast. Patients with diabetes took metformin or glibenclamide before the meal.

RESULTS

Systolic and diastolic blood pressure and plasma glucose were higher in both diabetic groups, for the whole experiment. Blood glucose did not change during exercise in the three groups and increased at recovery only in the control group. Plasma glucagon concentrations at the end of exercise and recovery, and plasma lactate concentrations at recovery were higher in the metformin group. Insulin, noradrenaline, growth hormone, cortisol and free fatty acid responses were similar in all three groups.

CONCLUSIONS

Our results suggest that the usual dose of glibenclamide and metformin can be taken safely before postprandial exercise of moderate intensity without affecting cardiovascular, metabolic and hormonal responses. However, after exercise, glibenclamide and metformin prevent the normal rise in blood glucose and metformin delays the fall in plasma lactate concentrations.

Activation of endothelial NADPH oxidase during normoxic lung ischemia is KATP channel dependent

Previous studies have shown endothelial cell membrane depolarization and generation of reactive oxygen species (ROS) in endothelial cells with abrupt reduction in shear stress (ischemia). This study evaluated the role of ATP-sensitive potassium (K(ATP)) channels and NADPH oxidase in the ischemic response by using Kir6.2-/- and gp91(phox)-/- mice. To evaluate ROS generation, we subjected isolated perfused mouse lungs labeled with 2',7'-dichlorodihydrofluorescein (DCF), hydroethidine (HE), or diphenyl-1-pyrenylphosphine (DPPP) to control perfusion followed by global ischemia. In wild-type C57BL/6J mice, imaging of subpleural endothelial cells showed a time-dependent increase in intensity for all three fluorescence probes with ischemia, which was blocked by preperfusion with cromakalim (a K(ATP) channel agonist) or diphenyleneiodonium (DPI, a flavoprotein inhibitor). Endothelial cell fluorescence with bis-oxonol, a membrane potential probe, increased during lung ischemia indicating cell membrane depolarization. The change in membrane potential with ischemia in lungs of gp91(phox)-/- mice was similar to wild type, but ROS generation did not occur. Lungs from Kir6.2-/- showed marked attenuation of the change in both membrane potential and ROS production. Thus membrane depolarization during lung ischemia requires the presence of a K(ATP) channel and is required for activation of NADPH oxidase and endothelial ROS generation.

Diethyl pyrocarbonate, a histidine-modifying agent, directly stimulates activity of ATP-sensitive potassium channels in pituitary GH(3) cells

The ATP-sensitive K(+) (K(ATP)) channels are composed of sulfonylurea receptor and inwardly rectifying K(+) channel (Kir6.2) subunit. These channels are regulated by intracellular ADP/ATP ratio and play a role in cellular metabolism. Diethyl pyrocarbonate (DEPC), a histidine-specific alkylating reagent, is known to modify the histidine residues of the structure of proteins. The objective of this study was to determine whether DEPC modifies K(ATP)-channel activity in pituitary GH(3) cells. Steady-state fluctuation analyses of macroscopic K(+) current at -120 mV produced power spectra that could be fitted with a single Lorentzian curve in these cells. The time constants in the presence of DEPC were increased. Consistent with fluctuation analyses, the mean open time of K(ATP)-channels was significantly increased during exposure to DEPC. However, DEPC produced no change in single-channel conductance, despite the ability of this compound to enhance K(ATP)-channel activity in a concentration-dependent manner with an EC(50) value of 16 microM. DEPC-stimulated K(ATP)-channel activity was attenuated by pretreatment with glibenclamide. In current-clamp configuration, DEPC decreased the firing of action potentials in GH(3) cells. A further application of glibenclamide reversed DEPC-induced inhibition of spontaneous action potentials. Intracellullar Ca(2+) measurements revealed the ability of DEPC to decrease Ca(2+) oscillations in GH(3) cells. Simulation studies also demonstrated that the increased conductance of K(ATP)-channels used to mimic DEPC actions reduced the frequency of spontaneous action potentials and fluctuation of intracellular Ca(2+). The results indicate that chemical modification with DEPC enhances K(ATP)-channel activity and influences functional activities of pituitary GH(3) cells.

Pore-forming subunits of K-ATP channels, Kir6.1 and Kir6.2, display prominent differences in regional and cellular distribution in the rat brain

K-ATP channels consist of two structurally different subunits: a pore-forming subunit of the Kir6.0-family (Kir6.1 or Kir6.2) and a sulfonylurea receptor (SUR1, SUR2, SUR2A, SUR2B) with regulatory activity. The functional diversity of K-ATP channels in brain is broad and of fundamental importance for neuronal activity. Here, using immunocytochemistry with monospecific antibodies against the Kir6.1 and Kir6.2 subunits, we analyze the regional and cellular distribution of both proteins in the adult rat brain. We find Kir6.2 to be widely expressed in all brain regions, suggesting that the Kir6.2 subunit forms the pore of the K-ATP channels in most neurons, presumably protecting the cells during cellular stress conditions such as hypoglycemia or ischemia. Especially in hypothalamic nuclei, in particular the ventromedial and arcuate nucleus, neurons display Kir6.2 immunoreactivity only, suggesting that Kir6.2 is the pore-forming subunit of the K-ATP channels in the glucose-responsive neurons of the hypothalamus. In contrast, Kir6.1-like immunolabeling is restricted to astrocytes (Thomzig et al. [2001] Mol Cell Neurosci 18:671-690) in most areas of the rat brain and very weak or absent in neurons. Only in distinct nuclei or neuronal subpopulations is a moderate or even strong Kir6.1 staining detected. The biological functions of these K-ATP channels still need to be elucidated.

KATP channel openers: structure-activity relationships and therapeutic potential

ATP-sensitive potassium channels (K(ATP) channels) are heteromeric complexes of pore-forming inwardly rectifying potassium channel subunits and regulatory sulfonylurea receptor subunits. K(ATP) channels were identified in a variety of tissues including muscle cells, pancreatic beta-cells, and various neurons. They are regulated by the intracellular ATP/ADP ratio; ATP induces channel inhibition and MgADP induces channel opening. Functionally, K(ATP) channels provide a means of linking the electrical activity of a cell to its metabolic state. Shortening of the cardiac action potential, smooth muscle relaxation, inhibition of both insulin secretion, and neurotransmitter release are mediated via K(ATP) channels. Given their many physiological functions, K(ATP) channels represent promising drug targets. Sulfonylureas like glibenclamide block K(ATP) channels; they are used in the therapy of type 2 diabetes. Openers of K(ATP) channels (KCOs), for example, relax smooth muscle and induce hypotension. KCOs are chemically heterogeneous and include as different classes as the benzopyrans, cyanoguanidines, thioformamides, thiadiazines, and pyridyl nitrates. Examples for new chemical entities more recently developed as KCOs include cyclobutenediones, dihydropyridine related structures, and tertiary carbinols.

The insulin secretagogues glibenclamide and repaglinide do not influence growth hormone secretion in humans but stimulate glucagon secretion during profound insulin deficiency

In vitro data have recently suggested that sulfonylureas (SUs) enhance GH secretion by modulating the effects of GHRH and somatostatin in pituitary cells. The present study was undertaken to explore in more detail a possible influence of a single dose of SU (glibenclamide) and a non-SU (repaglinide) insulin secretagogue on circulating GH dynamics. Ten C-peptide-negative type 1 diabetic individuals were examined on three occasions in random order. Either glibenclamide (10.5 mg), repaglinide (8 mg), or placebo was administered after overnight normalization of plasma glucose by iv insulin infusion. Subsequently, GH concentrations were measured regularly after stimulation with GHRH (bolus 0.1 micro g/kg) alone and during concomitant infusion with somatostatin (7 ng.kg(-1).min(-1)). Insulin was replaced at baseline levels (0.25 mU.kg(-1).min(-1)) and plasma glucose clamped at 5-6 mmol/liter. Overall, there were no significant statistical differences in GH responses determined as either GH peak concentrations, integrated levels of GH, or secretory burst mass of GH during the experimental protocol. In contrast, plasma glucagon concentrations were significantly increased during glibenclamide and repaglinide exposure. The present experimental design does not support the hypothesis that acute administration of pharmacological doses of the oral antihyperglycemic agents glibenclamide and repaglinide per se enhance GH release in humans. Additionally, this study shows that these potassium channel inhibitors seem to stimulate glucagon secretion in people who have severe intraislet insulin deficiency (e.g. type 1 diabetes). However, extrapolation of our findings to type 2 diabetic individuals should be done with some caution.

Cloning of a pore-forming subunit of ATP-sensitive potassium channel from Clonorchis sinensis

A complete cDNA sequence encoding a pore-forming subunit (Kir6.2) of ATP-sensitive potassium channel in the adult worm, Clonorchis sinensis, termed CsKir6.2, was isolated from an adult cDNA library. The cDNA contained a single open-reading frame of 333 amino acids, which has a structural motif (a GFG-motif) of the putative pore-forming loop of the Kir6.2. Peculiarly, the CsKir6.2 shows a lack-sequence structure, which deleted 57 amino acids were deleted from its N-terminus. The predicted amino acid sequence revealed a highly conserved sequence as other known other Kir6.2 subunits. The mRNA was weekly expressed in the adult worm.

Physiological and pathophysiological roles of ATP-sensitive K+ channels

ATP-sensitive potassium (K(ATP)) channels are present in many tissues, including pancreatic islet cells, heart, skeletal muscle, vascular smooth muscle, and brain, in which they couple the cell metabolic state to its membrane potential, playing a crucial role in various cellular functions. The K(ATP) channel is a hetero-octamer comprising two subunits: the pore-forming subunit Kir6.x (Kir6.1 or Kir6.2) and the regulatory subunit sulfonylurea receptor SUR (SUR1 or SUR2). Kir6.x belongs to the inward rectifier K(+) channel family; SUR belongs to the ATP-binding cassette protein superfamily. Heterologous expression of differing combinations of Kir6.1 or Kir6.2 and SUR1 or SUR2 variant (SUR2A or SUR2B) reconstitute different types of K(ATP) channels with distinct electrophysiological properties and nucleotide and pharmacological sensitivities corresponding to the various K(ATP) channels in native tissues. The physiological and pathophysiological roles of K(ATP) channels have been studied primarily using K(ATP) channel blockers and K(+) channel openers, but there is no direct evidence on the role of the K(ATP) channels in many important cellular responses. In addition to the analyses of naturally occurring mutations of the genes in humans, determination of the phenotypes of mice generated by genetic manipulation has been successful in clarifying the function of various gene products. Recently, various genetically engineered mice, including mice lacking K(ATP) channels (knockout mice) and mice expressing various mutant K(ATP) channels (transgenic mice), have been generated. In this review, we focus on the physiological and pathophysiological roles of K(ATP) channels learned from genetic manipulation of mice and naturally occurring mutations in humans.

Effect of diabetes on pinacidil-induced antinociception in mice

The antinociceptive effects of pinacidil, an adenosine triphosphate (ATP)-sensitive K(+)i (K(ATP)) channel opener, were examined using the tail-flick test in non-diabetic and diabetic mice. Pinacidil i.c.v. produced dose-dependent antinociception in both non-diabetic and diabetic mice. There was no significant difference between the antinociceptive effect of i.c.v. pinacidil in non-diabetic mice and diabetic mice. The i.t. administration of pinacidil also produced dose-dependent antinociception in both non-diabetic and diabetic mice, however, the antinociceptive effect of i.t. pinacidil in diabetic mice was significantly greater than that in non-diabetic mice. The antinociceptive effect of i.c.v. or i.t. pinacidil was significantly antagonized by i.c.v. or i.t. glibenclamide, a K(ATP) channel blocker in both non-diabetic and diabetic mice. In non-diabetic mice, the antinociceptive effect of i.c.v. or i.t. administration of pinacidil was significantly antagonized by beta-funaltrexamine, a mu-opioid receptor antagonist, 7-benzylidenenaltrexone, a delta1-opioid receptor antagonist, naltriben, a delta2-opioid receptor antagonist, and nor-binaltorphimine, a kappa-opioid receptor antagonist. In diabetic mice, the antinociceptive effect of i.c.v. pinacidil was significantly reduced by 7-benzylidenenaltrexone, naltriben, and nor-binaltorphimine. However, beta-funaltrexamine had no effect on antinociception induced by i.c.v. pinacidil in diabetic mice. On the other hand, the antinociceptive effect of i.t. pinacidil was significantly antagonized by beta-funaltrexamine, 7-benzylidenenaltrexone, naltriben, and nor-binaltorphimine in diabetic mice. These results indicated that pinacidil produced antinociception through the release of opioid peptides acting at mu-, delta- and kappa-opioid receptors in surpraspinal and spinal cord of non-diabetic mice. On the other hand, in diabetic mice, the antinociception-induced by pinacidil was mediated through the release of opioid peptides acting at delta- and kappa-opioid receptors supraspinally, whereas pinacidil produced antinociception through the release of opioid peptides acting at mu-, delta-, and kappa-opioid receptors spinally.

The sulfonylurea glyburide induces impairment of glucagon and growth hormone responses during mild insulin-induced hypoglycemia

OBJECTIVE

The sulfonylurea (SU) glyburide may cause severe and prolonged episodes of hypoglycemia. We aimed at investigating the impact of glyburide on glucose counterregulatory hormones during stepwise hypoglycemic clamp studies.

RESEARCH DESIGN AND METHODS

We performed stepwise hypoglycemic clamp studies in 16 healthy volunteers (7 women and 9 men aged 44 +/- 10 years). We investigated counterregulatory hormonal and symptom responses at arterialized venous plasma glucose levels (PG) of 3.8, 3.2, and 2.6 mmol/l, comparing 10 mg glyburide orally and placebo in a double-blind, randomized crossover fashion.

RESULTS

The increase in plasma glucagon with time from PG = 3.8 onward was smaller for glyburide than for placebo (P = 0.014). Plasma glucagon area under the curve (AUC)(60-180) was lower after glyburide than after placebo (1,774 +/- 715 vs. 2,161 +/- 856 pmol. l(-1). min, P = 0.014). From PG = 3.8 onward, plasma growth hormone (GH) levels with placebo were nearly two times (1.9 [95% CI 1.2-2.9]) as high as with glyburide (P = 0.011). AUC(60-180) for GH was lower after glyburide than after placebo (geometric mean [range] 665 [356-1,275] and 1,058 [392-1,818] mU. l(-1). min, respectively; P = 0.04). No significant differences were observed for plasma cortisol, epinephrine and norepinephrine, or incremental symptom scores.

CONCLUSIONS

The SU glyburide induces multiple defects in glucose counterregulatory hormonal responses, notably decreases in both glucagon and GH release.

ATP-sensitive K+ channels in the hypothalamus are essential for the maintenance of glucose homeostasis

Glucose-responsive (GR) neurons in the hypothalamus are thought to be critical in glucose homeostasis, but it is not known how they function in this context. Kir6.2 is the pore-forming subunit of K(ATP) channels in many cell types, including pancreatic beta-cells and heart. Here we show the complete absence of both functional ATP-sensitive K+ (K(ATP)) channels and glucose responsiveness in the neurons of the ventromedial hypothalamus (VMH) in Kir6.2-/- mice. Although pancreatic alpha-cells were functional in Kir6.2-/-, the mice exhibited a severe defect in glucagon secretion in response to systemic hypoglycemia. In addition, they showed a complete loss of glucagon secretion, together with reduced food intake in response to neuroglycopenia. Thus, our results demonstrate that KATP channels are important in glucose sensing in VMH GR neurons, and are essential for the maintenance of glucose homeostasis.

Analysis of single K(ATP) channels in mammalian dentate gyrus granule cells

ATP-sensitive potassium (K(ATP)) channels are heteromultimer complexes of subunits from members of the inwardly rectifying K(+) channel and the ATP-binding cassette protein superfamilies. K(ATP) channels couple metabolic state to membrane excitability, are distributed widely, and participate in a variety of physiological functions. Understood best in pancreatic beta cells, where their activation inhibits insulin release, K(ATP) channels have been implicated also in postischemia cardio- and neuroprotection. The dentate gyrus (DG) is a brain region with a high density of K(ATP) channels and is relatively resistant to ischemia/reperfusion-induced cell death. Therefore we were interested in describing the characteristics of single K(ATP) channels in DG granule cells. We recorded single K(ATP) channels in 59/105 cell-attached patches from DG granule cells in acutely prepared hippocampal slices. Single-channel openings had an E(K) close to 0 mV (symmetrical K(+)) and were organized in bursts with a duration of 19.3 +/- 1.6 (SE) ms and a frequency of 3.5 +/- 0.8 Hz, a unitary slope conductance of 27 pS, and a low, voltage-independent, probability of opening (P(open), 0.04 +/- 0.01). Open and closed dwell-time histograms were fitted best with one (tau(open) = 1.3 +/- 0.2 ms) and the sum of two (tau(closed,fast) = 2.6 +/- 0.9 ms, tau(closed,slow) = 302.7 +/- 67. 7 ms) exponentials, respectively, consistent with a kinetic model having at least a single open and two closed states. The P(open) was reduced ostensibly to zero by the sulfonylureas, glybenclamide (500 nM, 2/6; 10 microM,11/14 patches) and tolbutamide (20 microM, 4/6; 100 microM, 4/4 patches). The blocking dynamics for glybenclamide included transition to a subconductance state (43.3 +/- 2.6% of control I(open channel)). Unlike glybenclamide, the blockade produced by tolbutamide was reversible. In 5/5 patches, application of diazoxide (100 microM) increased significantly P(open) (0.12 +/- 0.02), which was attributable to a twofold increase in the frequency of bursts (8.3 +/- 2.0 Hz). Diazoxide was without effect on tau(open) and tau(closed,fast) but decreased significantly tau(closed,slow) (24.4 +/- 2.6 ms). We observed similar effects in 6/7 patches after exposure to hypoxia/hypoglycemia, which increased significantly P(open) (0.09 +/- 0.03) and the frequency of bursts (7.1 +/- 1.7 Hz) and decreased significantly tau(closed,slow) (29.5 +/- 1.8 ms). We have presented convergent evidence consistent with single K(ATP) channel activity in DG granule cells. The subunit composition of K(ATP) channels native to DG granule cells is not known; however, the characteristics of the channel activity we recorded are representative of Kir6.1/SUR1, SUR2B-based channels.

Pharmacological blockade of ERG K(+) channels and Ca(2+) influx through store-operated channels exerts opposite effects on intracellular Ca(2+) oscillations in pituitary GH(3) cells

In the present study, the effects on intracellular calcium concentration ([Ca(2+)](i)) oscillations of the blockade of ether-a-go-go-related gene (ERG) K(+) channels and of Ca(2+) influx through store-operated channels (SOC) activated by [Ca(2+)](i) store depletion have been studied in GH(3) cells by means of a combination of single-cell fura-2 microfluorimetry and whole-cell mode of the patch-clamp technique. Nanomolar concentrations (1-30 nM) of the piperidinic second-generation antihistamines terfenadine and astemizole and of the class III antiarrhythmic methanesulfonanilide dofetilide, by blocking ERG K(+) channels, increased the frequency and the amplitude of [Ca(2+)](i) oscillations in resting oscillating GH(3) cells. These compounds also induced the appearance of an oscillatory pattern of [Ca(2+)](i) in a subpopulation of nonoscillating GH(3) cells. The effects of ERG K(+) channel blockade on [Ca(2+)](i) oscillations appeared to be due to the activation of L-type Ca(2+) channels, because they were prevented by 300 nM nimodipine. By contrast, the piperazinic second-generation antihistamine cetirizine (0.01-30 microM), which served as a negative control, failed to affect ERG K(+) channels and did not interfere with [Ca(2+)](i) oscillations in GH(3) cells. Interestingly, micromolar concentrations of terfenadine and astemizole (0.3-30 microM), but not of dofetilide (10-100 microM), produced an inhibition of the spontaneous oscillatory pattern of [Ca(2+)](i) changes. This effect was possibly related to an inhibition of SOC, because these compounds inhibited the increase of [Ca(2+)](i) achieved by extracellular calcium reintroduction after intracellular calcium store depletion with the sarcoplasmic or endoplasmic reticulum calcium ATPase pump inhibitor thapsigargin (10 microM) in an extracellular calcium-free medium. The same inhibitory effect on [Ca(2+)](i) oscillations and SOC was observed with the first-generation antihistamine hydroxyzine (1-30 microM), the more hydrophobic metabolic precursor of cetirizine. Collectively, the results of the present study obtained with compounds that interfere in a different concentration range with ERG K(+) channels or SOC suggest that 1) ERG K(+) channels play a relevant role in controlling the oscillatory pattern of [Ca(2+)](i) in resting GH(3) cells and 2) the inhibition of SOC might induce an opposite effect, i.e., an inhibition of [Ca(2+)](i) oscillations.

Endocrine disorders in two sisters affected by MELAS syndrome

A variety of endocrine and metabolic defects, including hypothalamopituitary hypofunction and diabetes mellitus, has been reported in association with mitochondrial disorders. We describe two sisters affected by mitochondrial encephalomyopathy, lactic acidosis, and strokelike episodes (MELAS) syndrome in whom DNA analysis showed an A-->G transition at the 3243rd nucleotide position on the transfer RNALeu(UUR) gene with 65% and 45% of mutant-type mitochondrial DNA present in the blood cells of the younger and the older sister, respectively. The younger sister had severe involvement of the central nervous system with mental retardation, epilepsia partialis continua, and strokelike episodes. Endocrine investigations showed an extensive neuroendocrine dysfunction with growth hormone deficiency, hypothalamopituitary hypothyroidism, prepubertal gonadotropin levels, and absence of any secondary sexual characteristics at the age of 12 6/12 years. The neurologically normal older sister was affected by diabetes mellitus and had normal hypothalamopituitary function. Our report confirms that the endocrine system can be affected differently by the same mitochondrial DNA mutation, depending on the heteroplasmia phenomenon. A complete endocrine evaluation must be performed in patients affected by mitochondrial disease and the existence of a mitochondrial disorder should be taken into account in patients with endocrine abnormalities, even if neuromuscular signs are lacking.

Intercellular communication in the anterior pituitary

In addition to hypothalamic and feedback inputs, the secretory cells of the anterior pituitary are influenced by the activity of factors secreted within the gland. The list of putative intrapituitary factors has been expanding steadily over the past decade, although until recently much of the work was limited to descriptions of potential interactions. This took the form of evidence of production within the pituitary of factors already known to influence activity of secretory cells, or further descriptions of actions on pituitary cells by such factors when added exogenously. A new phase of discovery has been entered, with extensive efforts being made to delineate the control of the synthesis and secretion of the pituitary factors within the gland, regulation of the receptors and response mechanisms for the factors in pituitary cells, and measurements of the endogenous actions of the factors through the use of specific immunoneutralization, receptor blockade, tissue from transgenic animals, and other means. Taken together, these findings are producing blueprints of the intrapituitary interactions that influence each of the individual types of secretory cells, leading toward an understanding of the physiological significance of the interactions. The purpose of this article is to review the recent literature on many of the factors acting as intrapituitary signals and to present such finding in the context of the physiology of the secretory cells.

Expression of mRNA for voltage-dependent and inward-rectifying K channels in GH3/B6 cells and rat pituitary

The expression of mRNA for voltage-dependent (Kv) and inward-rectifying K channels (Kir) was studied in clonal rat somato-mammotroph cells (GH3/B6 cells) and rat pituitary using reverse transcription-polymerase chain reaction (RT-PCR). In GH3/B6 cells transcripts for 16 different Kv channel alpha-subunits (seven Shaker-related: Kv1.2, Kv1.4, Kv1.5, Kv2.1, Kv3.2, Kv4.1, Kv5.1; six EAG: eag1, erg1, erg2, elk1-elk3; three KCNQ: KCNQ1-KCNQ3) and for five different Kir channel alpha-subunits (Kir1.1, Kir2.3, Kir3.2, Kir3.3, Kir6.2) were found. In addition, transcripts for a short isoform of Kvbeta2 and transcripts for Kvbeta3 subunits were present. In rat pituitary transcripts for 21 different Kv channel alpha-subunits (11 Shaker-related: Kv1.3, Kv1.4, Kv1.6, Kv2.1, Kv2.2, Kv3.2, Kv3.4, Kv4.1, Kv4.2, Kv4.3, Kv6.1; seven EAG: eag1, erg1-erg3, elk1-elk3; three KCNQ: KCNQ1-KCNQ3) and nine Kir channel alpha-subunits (Kir1.1, Kir2.2, Kir3.1-Kir3.4, Kir4.1, Kir6.1, Kir6. 2) were found. In addition, all tested auxiliary subunits (Kvbeta1-Kvbeta3, minK, SUR1, SUR2) are expressed in the pituitary. The results indicate that the macroscopic K currents in GH3/B6 and pituitary cells are presumably mediated by K channels constructed by a larger number of K channel alpha-subunits and auxiliary beta-subunits than previously distinguished electrophysiologically and pharmacologically.

Rhythmic bursts of calcium transients in acute anterior pituitary slices

Endocrine cells isolated from the anterior pituitary fire intracellular Ca2+ ([Ca2+]i) transients due to voltage-gated Ca2+ entry. However, the patterns of [Ca2+]i transients within the glandular parenchyma of the anterior pituitary are unknown. Here we describe, using real-time confocal laser microscopy, several spontaneous patterns of calcium signaling in acute pituitary slices prepared from male as well as cycling and lactating female rats. Forty percent of the cells demonstrated a spontaneous bursting mode, consisting of an active period of [Ca2+]i transients firing at a constant frequency, followed by a rest period during which cells were either silent or randomly active. The remaining recordings from endocrine cells either demonstrated random [Ca2+]i transients or were silent. These rhythmic bursts of [Ca2+]i transients, which required extracellular calcium, were detected in lactotrophs, somatotrophs, and corticotrophs within the acute slices. Of significance was the finding that the bursting mode could be adjusted by hypothalamic factors. In slices prepared from lactating rats, TRH recruited more bursting cells and finely adjusted the average duty cycle of [Ca2+]i bursts such that cells fired patterned bursts for approximately 70% of the recording period. Eighty-six percent of these cells were lactotrophs. Thus, the rhythmic [Ca2+]i bursts and their tuning by secretagogues may provide timing information that could encode for one or more cellular functions (e.g. exocytosis and/or gene expression) critical for the release of hormones by endocrine cells in the intact gland.

A novel K(ATP) current in cultured neonatal rat atrial appendage cardiomyocytes

The functional and pharmacological properties of ATP-sensitive K(+) (K(ATP)) channels were studied in primary cultured neonatal rat atrial appendage cardiomyocytes. Activation of a whole-cell inward rectifying K(+) current depended on the pipette ATP concentration and correlated with a membrane hyperpolarization close to the K(+) equilibrium potential. The K(ATP) current could be activated either spontaneously or by a hypotonic stretch of the membrane induced by lowering the osmolality of the bathing solution from 290 to 260 mOsm/kg H(2)O or by the K(+) channel openers diazoxide and cromakalim with EC(50) approximately 1 and 10 nmol/L, respectively. The activated atrial K(ATP) current was highly sensitive to glyburide, with an IC(50) of 1.22+/-0.15 nmol/L. Recorded in inside-out patches, the neonatal atrial K(ATP) channel displayed a conductance of 58.0+/-2.2 pS and opened in bursts of 133.8+/-20.4 ms duration, with an open time duration of 1.40+/-0.10 ms and a close time duration of 0.66+/-0.04 ms for negative potentials. The channel had a half-maximal open probability at 0.1 mmol/L ATP, was activated by 100 micromol/L diazoxide, and was inhibited by glyburide, with an IC(50) in the nanomolar range. Thus, pending further tests at low concentrations of K(ATP) channel openers, the single-channel data confirm the results obtained with whole-cell recordings. The neonatal atrial appendage K(ATP) channel thus shows a unique functional and pharmacological profile resembling the pancreatic beta-cell channel for its high affinity for glyburide and diazoxide and for its conductance, but also resembling the ventricular channel subtype for its high affinity for cromakalim, its burst duration, and its sensitivity to ATP. Reverse transcriptase-polymerase chain reaction experiments showed the expression of Kir6.1, Kir6.2, SUR1A, SUR1B, SUR2A, and SUR2B subunits, a finding supporting the hypothesis that the neonatal atrial K(ATP) channel corresponds to a novel heteromultimeric association of K(ATP) channel subunits.

ATP-sensitive potassium channels: a model of heteromultimeric potassium channel/receptor assemblies

ATP-sensitive K+ channels (KATP channels) play important roles in many cellular functions by coupling cell metabolism to electrical activity. By cloning members of the novel inwardly rectifying K+ channel subfamily Kir6.0 (Kir6.1 and Kir6.2) and the receptors for sulfonylureas (SUR1 and SUR2), researchers have clarified the molecular structure of KATP channels. KATP channels comprise two subunits: a Kir6.0 subfamily subunit, which is a member of the inwardly rectifying K+ channel family; and a SUR subunit, which is a member of the ATP-binding cassette (ABC) protein superfamily. KATP channels are the first example of a heteromultimeric complex assembled with a K+ channel and a receptor that are structurally unrelated to each other. Since 1995, molecular biological and molecular genetic studies of KATP channels have provided insights into the structure-function relationships, molecular regulation, and pathophysiological roles of KATP channels.

ATP-sensitive potassium channels: structures, functions, and pathophysiology

ATP-sensitive potassium channels (KATP channels) play important roles in various tissues by coupling cell metabolic status to electrical activity. Recently, molecular biological and electrophysiological techniques have revealed the molecular basis of the KATP channels to be a complex of the Kir6.0 subunit, a member of the inwardly rectifying K+ channel subfamily Kir6.0, and the sulfonylurea receptor (SUR) subunit, a member of ATP-binding cassette (ABC) superfamily; the functional diversity of the various KATP channels is being determined by a combination of the Kir6.0 subunit (Kir6.1 or Kir6.2) and the SUR subunit (SUR1 or SUR2) comprising it. Recent studies of the KATP channels have suggested mechanisms of KATP channel regulation and pathophysiology and also a new model in which ABC proteins regulate the functional expression of ion channels.

Sulfonylurea receptor mRNA expression in pituitary macroadenomas

ATP-sensitive K+ (KATP) channels modulated by sulfonylurea compounds have been previously identified in the anterior pituitary of the rat and have been demonstrated to influence GH release. Recently, a sulfonylurea receptor (SUR) has been cloned from an islet cell tumor and identified as a member of the ATP binding cassette superfamily capable to coupling with inwardly rectifying potassium channels. To determine if the same receptor is expressed in pituitary tumors, SUR mRNA levels were measured in 28 human macroadenoma specimens using an RNase protection assay. All immunonegative, corticotrophin (ACTH), growth hormone (GH), and GH/prolactin (GH/Prl) immunostaining tumors expressed detectable amounts of SUR message. Among these tumors, only the GH and GH/ Prl adenomas were functional. Of the tumors immunostaining for luteinizing hormone (LH), follicle-stimulating hormone (FSH), or both, SUR mRNA was present in small amounts in 5/11. Only 1/3 Prl immunostaining tumors contained SUR mRNA. In summary, we have demonstrated that SUR mRNA expression is common in several types of silent pituitary adenomas and in functional tumors that secrete GH. Lower levels are seen in some gonadotrophin immunostaining tumors.

ATP-sensitive K+ channels in pancreatic, cardiac, and vascular smooth muscle cells

ATP-sensitive K+ (KATP) channels are therapeutic targets for several diseases, including angina, hypertension, and diabetes. This is because stimulation of KATP channels is thought to produce vasorelaxation and myocardial protection against ischemia, whereas inhibition facilitates insulin secretion. It is well known that native KATP channels are inhibited by ATP and sulfonylurea (SU) compounds and stimulated by nucleotide diphosphates and K+ channel-opening drugs (KCOs). Although these characteristics can be shared with KATP channels in different tissues, differences in properties among pancreatic, cardiac, and vascular smooth muscle (VSM) cells do exist in terms of the actions produced by such regulators. Recent molecular biology and electrophysiological studies have provided useful information toward the better understanding of KATP channels. For example, native KATP channels appear to be a complex of a regulatory protein containing the SU-binding site [sulfonylurea receptor (SUR)] and an inward-rectifying K+ channel (Kir) serving as a pore-forming subunit. Three isoforms of SUR (SUR1, SUR2A, and SUR2B) have been cloned and found to have two nucleotide-binding folds (NBFs). It seems that these NBFs play an essential role in conferring the MgADP and KCO sensitivity to the channel, whereas the Kir channel subunit itself possesses the ATP-sensing mechanism as an intrinsic property. The molecular structure of KATP channels is thought to be a heteromultimeric (tetrameric) assembly of these complexes: Kir6.2 with SUR1 (SUR1/Kir6.2, pancreatic type), Kir6.2 with SUR2A (SUR2A/ Kir6.2, cardiac type), and Kir6.1 with SUR2B (SUR2B/Kir6.1, VSM type) [i.e., (SUR/Kir6.x)4]. It remains to be determined what are the molecular connections between the SUR and Kir subunits that enable this unique complex to work as a functional KATP channel.

Abnormalities of pancreatic islets by targeted expression of a dominant-negative KATP channel

ATP-sensitive K+ (KATP) channels are known to play important roles in various cellular functions, but the direct consequences of disruption of KATP channel function are largely unknown. We have generated transgenic mice expressing a dominant-negative form of the KATP channel subunit Kir6.2 (Kir6.2G132S, substitution of glycine with serine at position 132) in pancreatic beta cells. Kir6.2G132S transgenic mice develop hypoglycemia with hyperinsulinemia in neonates and hyperglycemia with hypoinsulinemia and decreased beta cell population in adults. KATP channel function is found to be impaired in the beta cells of transgenic mice with hyperglycemia. In addition, both resting membrane potential and basal calcium concentrations are shown to be significantly elevated in the beta cells of transgenic mice. We also found a high frequency of apoptotic beta cells before the appearance of hyperglycemia in the transgenic mice, suggesting that the KATP channel might play a significant role in beta cell survival in addition to its role in the regulation of insulin secretion.

Developmental and tissue-specific sulfonylurea receptor gene expression

We have studied the developmental regulation of mouse sulfonylurea receptor (SUR) gene expression throughout several embryonic stages as well as in the adult mouse. To this end we used a 229-bp mouse complementary DNA corresponding to the 3'-end of the SUR gene for in situ hybridization and solution hybridization/ribonuclease protection assays. We found that the SUR gene was expressed as early as embryonic day 12 in the developing pancreas, heart, and central nervous system. These tissues maintained significant levels of SUR messenger RNA (mRNA) throughout development. In addition, SUR mRNA was detected in the submandibular gland, anterior duodenum, dorsal root ganglia, lens, retina, and vibrissae by late developmental stages. SUR mRNA is widely distributed in adult mouse tissues, with the exception of the liver. In the adult pancreas, the SUR gene was expressed exclusively in endocrine tissue. Although significant levels of SUR mRNA were broadly seen throughout the brain, neurons of the cerebellum, hippocampus, and thalamus had especially high levels of SUR mRNA. These findings support the idea that the SUR has important functions in many other tissues in addition to the islets of the pancreas.

ATP-sensitive K+ channels in the kidney

ATP-sensitive K+ channels (KATP channels) form a link between the metabolic state of the cell and the permeability of the cell membrane for K+ which, in turn, is a major determinant of cell membrane potential. KATP channels are found in many different cell types. Their regulation by ATP and other nucleotides and their modulation by other cellular factors such as pH and kinase activity varies widely and is fine-tuned for the function that these channels have to fulfill. In most excitable tissues they are closed and open when cell metabolism is impaired; thereby the cell is clamped in the resting state which saves ATP and helps to preserve the structural integrity of the cell. There are, however, notable exceptions from this rule; in pancreatic beta-cells, certain neurons and some vascular beds, these channels are open during the normal functioning of the cell. In the renal tubular system, KATP channels are found in the proximal tubule, the thick ascending limb of Henle's loop and the cortical collecting duct. Under physiological conditions, these channels have a high open probability and play an important role in the reabsorption of electrolytes and solutes as well as in K+ homeostasis. The physiological role of their nucleotide sensitivity is not entirely clear; one consequence is the coupling of channel activity to the activity of the Na-K-ATPase (pump-leak coupling), resulting in coordinated vectorial transport. In ischemia, however, the reduced ATP/ADP ratio would increase the open probability of the KATP channels independently from pump activity; this is particularly dangerous in the proximal tubule, where 60 to 70% of the glomerular ultrafiltrate is reabsorbed. The pharmacology of KATP channels is well developed including the sulphonylureas as standard blockers and the structurally heterogeneous family of channel openers. Blockers and openers, exemplified by glibenclamide and levcromakalim, show a wide spectrum of affinities towards the different types of KATP channels. Recent cloning efforts have solved the mystery about the structure of the channel: the KATP channels in the pancreatic beta-cell and in the principal cell of the renal cortical collecting duct are heteromultimers, composed of an inwardly rectifying K+ channel and sulphonylurea binding subunit(s) with unknown stoichiometry. The proteins making up the KATP channel in these two cell types are different (though homologous), explaining the physiological and pharmacological differences between these channel subtypes.

Nephropathy and growth hormone deficiency in a patient with mitochondrial tRNA(Leu(UUR)) mutation

A mitochondrial A 3243 G mutation in the tRNA(Leu(UUR)) gene was first described as a common cause of MELAS syndrome (mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like syndrome). This same mutation is also the cause of a totally different disorder, a subtype of diabetes mellitus which is inherited maternally and often associated with sensorineural hearing loss. In this paper, we report on a Japanese boy with A 3243 G who developed a previously undescribed combination of symptoms, nephropathy and growth hormone deficiency. The patient first presented with short stature and moderate mental retardation. Growth hormone (GH) provocation tests showed deficient growth hormone secretion. During the course of follow up, he presented with progressive nephropathy followed by the development of diabetes mellitus. The results of laboratory tests and renal biopsy were against incidental association of known types of nephropathy. On PCR-RFLP analysis, the percentage of mutated mtDNA was higher in the renal biopsy specimen than 12 peripheral blood leucocytes. Our case suggests that mitochondrial diseases should be taken into account when there is nephropathy of unknown cause. In addition, the presence of growth hormone deficiency may account for part of the mechanism leading to short stature commonly seen in these patients.

PKC-dependent stimulation of exocytosis by sulfonylureas in pancreatic beta cells

Hypoglycemic sulfonylureas represent a group of clinically useful antidiabetic compounds that stimulate insulin secretion from pancreatic beta cells. The molecular mechanisms involved are not fully understood but are believed to involve inhibition of potassium channels sensitive to adenosine triphosphate (KATP channels) in the beta cell membrane, causing membrane depolarization, calcium influx, and activation of the secretory machinery. In addition to these effects, sulfonylureas also promoted exocytosis by direct interaction with the secretory machinery not involving closure of the plasma membrane KATP channels. This effect was dependent on protein kinase C (PKC) and was observed at therapeutic concentrations of sulfonylureas, which suggests that it contributes to their hypoglycemic action in diabetics.

Coexistence of two classes of glibenclamide-inhibitable ATP-regulated K+ channels in avian skeletal muscle

Avian skeletal muscle expresses two types of ATP-sensitive K+ channels which have a unitary conductance of 15pS. These K+ channels can be distinguished pharmacologically by their high or low sensitivity to the antidiabetic sulphonylurea blocker glibenclamide. Both channels are activated by the K+ channel opener cromakalim. Chick skeletal muscle expresses high-affinity binding sites for [3H]glibenclamide (Kd = 0.6nM) which presumably correspond to the ATP-sensitive K+ channels with the greatest sensitivity to glibenclamide. The density of these high-affinity binding sites varies during muscle development. The maximum density (500fmol/mg protein) appears at 16 days in ovo, i.e. at a period when myoblasts have differentiated into myotubes and when innervation of myotubes has started. After this maximum, the level of [3H]glibenclamide-binding sites decreases to a plateau value of 100fmol/mg protein at 2-5 days post-natal. When muscle cells are put in cultures, the high-affinity binding sites disappear rapidly. Neither glibenclamide nor cromakalim have any effect on normal physiological chick muscle contraction. They have no effect on contracture and/or 86Rb+ efflux produced by metabolic poisoning.

Cardiac ATP-sensitive K+ channels: regulation by intracellular nucleotides and K+ channel-opening drugs

ATP-sensitive K+ (KATP) channels are present at high density in membranes of cardiac cells where they regulate cardiac function during cellular metabolic impairment. KATP channels have been implicated in the shortening of the action potential duration and the cellular loss of K+ that occurs during metabolic inhibition. KATP channels have been associated with the cardioprotective mechanism of ischemia-related preconditioning. Intracellular ATP (ATPi) is the main regulator of KATP channels. ATPi has two functions: 1) to close the channel (ligand function) and 2) in the presence of Mg2+, to maintain the activity of KATP channels (presumably through an enzymatic reaction). KATP channel activity is modulated by intracellular nucleoside diphosphates that antagonize the ATPi-induced inhibition of channel opening or induce KATP channels to open. How nucleotides will affect KATP channels depends on the state of the channel. K+ channel-opening drugs are pharmacological agents that enhance KATP channel activity through different mechanisms and have great potential in the management of cardiovascular conditions. KATP channel activity is also modulated by neurohormones. Adenosine, through the activation of a GTP-binding protein, antagonizes the ATPi-induced channel closure. Understanding the molecular mechanisms that underlie KATP channel regulation should prove essential to further define the function of KATP channels and to elucidate the pharmacological regulation of this channel protein. Since the molecular structure of the KATP channel has now become available, it is anticipated that major progress in the KATP channel field will be achieved.

Cloning and functional characterization of a novel ATP-sensitive potassium channel ubiquitously expressed in rat tissues, including pancreatic islets, pituitary, skeletal muscle, and heart

ATP-sensitive K+ (KATP) channels play a crucial role in coupling metabolic energy to the membrane potential of cells. We have isolated a cDNA encoding a novel member (uKATP-1) of the inward rectifier K+ channel family from a rat pancreatic islet cDNA library. Rat uKATP-1 is a 424-amino acid residue protein (M(r) = 47,960). Electrophysiological studies of uKATP-1 expressed in Xenopus laevis oocytes show that uKATP-1 is a weak rectifier and is blocked with Ba2+ ions. Single-channel patch clamp study of clonal human kidney epithelial cells (HEK293) transfected with uKATP-1 cDNA reveals that uKATP-1 closes in response to 1 mM ATP and has a single channel conductance of 70 +/- 2 picosiemens (n = 6), indicating that uKATP-1 is an ATP-sensitive inward rectifier K+ channel. In addition, uKATP-1 is activated by the KATP channel opener, diazoxide. RNA blot analysis shows that uKATP-1 mRNA is expressed ubiquitously in rat tissues, including pancreatic islets, pituitary, skeletal muscle, and heart, suggesting that uKATP-1 may play a physiological role as a link between the metabolic state and membrane K+ permeability of cells in almost every normal tissue. Since uKATP-1 shares only 43-46% amino acid identity with members of previously reported inward rectifier K+ channel subfamilies, including ROMK1, IRK1, GIRK1, and cKATP-1, uKATP-1 is not an isoform of these subfamilies and, therefore, represents a new subfamily of the inward rectifier K+ channel family having two transmembrane segments.

Potassium channel openers and other regulators of KATP channels

1. Interest in ATP-sensitive K (KATP) channels first arose when it was shown that hypoglycaemic sulphonylureas, such as glibenclamide, closed these channels in pancreatic beta-cells to cause insulin release. The demonstration that certain smooth muscle relaxants (K channel openers) may exert their actions through opening a similar channel in vascular smooth muscle fueled further investigation of these channels and their physiological role in a variety of tissue types, including various types of smooth muscle, cardiac and skeletal muscle and neural and endocrine organ function. 2. The K channel openers have a variety of potential therapeutic applications, including disorders of smooth muscle hyperreactivity, such as hypertension, and a great deal of research has focused on this field. More recently, attention has turned to the cardiac actions of these compounds and this area is discussed in detail. One of the current problems is the lack of selectivity of KATP channel regulators. However, there have been a number of recent encouraging reports suggesting that, under certain pathophysiological conditions, the action of the K channel openers may be enhanced, conferring upon them some degree of selectivity. 3. A number of endogenous regulators of these channels have been identified, particularly in the category of endogenous openers of these channels. At present though, the physiological role of these channels and the endogenous regulators identified, is unclear. 4. It is evident that, although advances have been made, much work is still required to increase our understanding and ultimately to allow selective pharmacological manipulation of these channels to become a therapeutic reality.